The graph on the Home page shows the daily average for successive days. If we consider a single point in that graph, let’s say for example the value for the day 15 Jul 2011, its value is plotted as 0.138 μSv/h. This is the average for the whole day. The Geiger counter instrument utilized for this web site makes a measurement every 10 seconds, so in one day there are 8,640 measurements. Of course they are not all identical, and the graph in this page shows the histogram of all those readings. Each bin represents one of the possible readings provided by the instruments. Notice that 0.138 is never returned directly: it is calculate as an average figure.
If we consider as an example the highest bin, the third one from the left, it indicates that 22% of the times the reading was 0.097 μSv/h.
This shape resembles a Poisson distribution. I would like to know from some of you experts out there if the shape of this measured distribution is characteristic of the gamma radiation, so I am inviting you to post your comments.